1. Field
The present invention relates generally to actuators, and more particularly to hydraulic actuator devices suited for use in land vehicle suspension systems.
2. Description of Related Art
The use of servo actuators to control motion in land vehicle suspension systems offers the potential for improving performance, especially for off road application. For example, developers of military vehicles have been particularly interested in this technology to enable faster movement of military vehicles over rough battlefield terrain. Active suspension systems provide a smoother ride and greater maneuvering stability in both off-road and on-road vehicles than passive suspension systems.
Linear electromechanical actuators have been employed in active suspension system designs. Typical linear electromechanical actuators for active suspension systems consist of a rotary electric motor, an efficient means of providing mechanical advantage and converting rotary motion to linear motion, and a means of controlling the torque of the motor. For example, in one example of an active suspension electromechanical actuator, the mechanical advantage is a set of reduction gears and a rack and pinion pair to convert the rotary electromechanical torque to linear force. Typically, the linear actuator is supplemented by a passive spring (steel coil, air, or other) to support vehicle static weight. Although this combination rack and pinion based electromechanical actuator and supplemental spring works well, it is relatively long (poor ratio of actuator stroke to actuator length). This makes the rack and pinion actuator difficult to integrate into existing land vehicle suspension systems.
An alternate design approach utilizes hydraulic servo systems. Typical hydraulic servo systems have an electric motor, a hydraulic pump, a fluid reservoir, servo valves, flow passages, a hydraulic cylinder, and an array of pressure sensors and limit switches to control motion. This arrangement generally provides a good range of motion and can be configured to operate in all four control quadrants. The servo valve throttles the flow of fluid to control cylinder force, and in its best form controls fluid flow rate to the cylinder in a proportional manner by partially opening or throttling. In four quadrant systems, four-way servo valves are used to direct hydraulic pressure to one side of the cylinder and to vent the other side to the fluid reservoir. This arrangement is satisfactory in systems where motion is intermittent and where duty cycle is relatively low, such as construction equipment. In systems with high duty cycles, the conventional servo system offers low efficiency due to two major sources of losses. The first source of energy loss is in the servo valve, where throttling action is lossy, reducing pressure by dissipating energy. The second source of energy loss is in supply pressure regulation, typically accomplished by bleeding off or bypassing some of the flow to the reservoir to control the output pressure from the hydraulic pump. In systems where energy efficiency is important, pressure regulation is accomplished with a variable displacement hydraulic pump. However, this increased efficiency is at the expense of size, complexity and cost.